Tin oxide gas sensors

ABSTRACT

Tin oxide sensors are made by mixing antimony bearing material with tin oxide powder and formation of the sensor by deposition of a slurry of the mixture onto a substrate and drying and sintering the slurry, the antimony bearing material being present in an amount sufficient to render the sensitivity of the sensor to one or more of the gases H 2 , CO, or CH 4  , relatively independent of the concentration of oxygen in the range P O2  10 -1  -1 atm. A further type of a tin oxide gas sensor is disclosed having a resistivity that at a measuring temperature increases with concentration of at least one gas to be measured, the sensor is made by calcining the tin oxide in air at a temperature in excess of 1400° C., or otherwise treating the tin oxide so that it has a state of physical aggregation consistent with being formed in such manner. At a second measuring temperature the resistivity of the sensor to said one gas decreases with increasing gas concentration. The resistivity of the sensor is dependent on the concentration of several gases, the dependence at differing measuring temperatures being such that by measuring the resistivity of the sensor at several different measuring temperatures the composition of a gas to which the sensor is exposed may be calculated. An array of such tin oxide gas sensors may be mounted on a single substrate having heater means to maintain the sensors at differing temperatures. Such an array may also include an antimony bearing sensor as disclosed or other tin oxide sensors.

This invention relates to gas sensors of the type in which theresistance, or other electrical property, of a sample of tin (IV) oxide(SnO₂) is measured, the resistance, or other electrical property, beingdependent on the concentration of the gas in the surrounding medium. Inthe following reference is made to measurement of resistivity but itshould be understood that the invention is not restricted to suchmeasurement. At the rear of this description is a list of prior artrelating to tin oxide sensors and reference numerals in the descriptionrefers to this list of prior art.

Tin (IV) oxide (SnO₂) is widely used as the basis of solid state sensorscapable of detecting a variety of toxic and flammable gases [1-7]. Tin(IV) oxide is an n-type semiconductor in which electrical conductivityoccurs through negative charge carriers.

The active sensing element usually consists of a sinteredpolycrystalline mass of the oxide as exemplified by the many forms ofthe Figaro gas sensor produced in Japan. The fabrication procedureadopted involved heat treatment of the SnO₂ along with any otheradditives such as PdCI₂ or ThO₂ [2, 5, 6], which are initially dispersedin an aqueous slurry. This sintering process yields a sensor body ofsuitable mechanical strength and also confers thermal stability, whichis essential considering the elevated temperatures (300° C.-400° C.) atwhich these devices are operated. In order to achieve the desiredthermal stability, the sintering temperature (T_(S)) used must besignificantly higher than the sensor operating temperature (T_(o)).Typical values of T_(S) quoted in the literature for commerciallyavailable devices lie in the range 500°-700° C. [2, 6, 7]. However, itis widely known that tin oxide alone sinters poorly at thesetemperatures. Values of T_(S) exceeding 1100° C., which marks the Tammantemperature of the material [8], are required to achieve acceleratedadhesion between neighbouring crystallites. To improve low temperatureinter-granular cementing, binders such as tetraethyl ortho-silicate(TEOS) [9], MgO [6]or alumina [10] are often incorporated prior to heattreatment. These binders may significantly alter the gas sensingcharacteristics of the material, for example, in the case of TEOS, whichdecomposes at elevated temperatures forming Si--O bridges between theSnO₂ grains, the presence of the binder confers a marked increase insensitivity to flammable gases [9].

Considering the importance of the heat treatment step in the overallfabrication procedure, comparatively few studies have been performed onthe influence of sintering temperature on the characteristics of SnO₂based gas sensors. Research carried out by Borand [11] on pressedpellets of polycrystalline SnO₂ annealed in the 400° C.-900° C. rangeshowed that maximum CO sensitivity along with the shortest response timeoccurred at a sintering temperature of 700° C. However, for a tin oxidecombustion monitoring device Sasaki and his co-workers [12] found that asintering temperature of 1300° C. gave the most desirablecharacteristics.

Such sensors have been widely described and are usually in the form of athin or thick film deposit of the tin oxide on an alumina or otherinsulating ceramic substrate. Platinum paste contacts are used toconnect the tin oxide to wires for resistance measurement and anelectrical resistance heating element may be provided on the substrate.

Tin oxide sensors suffer a major drawback in that they are sensitive tomany gases and worse there are also some cross-sensitivities, i.e. thepresence of one gas will alter the sensitivity of the sensor to thepresence of a second gas.

A notable cross-sensitivity is the influence of oxygen at low oxygenpartial pressures. It is found that an undoped SnO₂ sensor experienceslarge changes in resistance (greater than three orders of magnitude)upon exposure to gases such as CO or H₂ under conditions of reducedoxygen partial pressure.

A further problem in the manufacture of such tin oxide gas sensor iscontrolling the resistivity of the sensor so as to be readilymeasurable. It is well-known to add antimony oxide (Sb₂ O₃) as an aid toreducing the base resistance of the tin oxide sensor. In a paper inSensors and Actuators 12 (1987) pages 77-89 [15], L. N. Yannopoulosdescribed the use of Sn₀.98 Sb₀.02 O₂. These researches indicated (pages86, 87) that the gas sensor response to H₂ was affected by changes inthe partial pressure of oxygen over the range 0.25-2.0%.

European Patent specification No. 0147213 (Westinghouse ElectricCorporation) discloses antimony doped tin oxide sensors containing0.5-2.5% w/w antimony formed by co-precipitation of tin and antimonymixed hydroxide from an admixture of stannic chloride liquid andantimony pentachloride liquid. The sensors produced were suitable fordetection of CO and H₂ but showed a marked oxygen cross-sensitivity (seeFIGS. 4 & 5).

United Kingdom 1596095 discloses use of antimony at a very low level(<0.1% w/w) and is directed to use of such sensors as gas alarms anddoes not address the problem of oxygen sensitivity at low oxygen partialpressures.

European No. 0114310 discloses use of antimony and platinum in a tinoxide sensor, the ratio of antimony to tin being 2:8 mole % and theratio of platinum to tin being in the ratio of 2:10 mole %. Thisspecification discloses very high levels of platinum and makes nomention of the suppression of of oxygen dependence. The main differencebetween this sensor and other sensors is that it is worked at roomtemperature and so has a very long response time. The reason why theantimony and platinum was added was probably so as to obtain aresistance which is low enough to measure at ambient temperatures. A bigproblem with room-temperature sensors is that they tend to be moreaffected by changing humidity than gas composition. Further the processfor forming the disclosed sensor involves firing at a temperature in therange 600°-850° C.

European Patent Specification No. 0261275 is to a hydrogen sensor usingantimony in the ratio 0:8 mole % but gives no mention whatsoever ofperformance of the sensor in oxygen deficient conditions. The disclosedmethod is similar to that in European 0114310.

European 0280540 is to a sensor comprising an antimony doped tin oxidesensing medium with a filter to vary the performance of the sensor.

As will be clear from the above there are many workers in the field oftin oxide gas sensors yet none have appreciated the virtues of thepresent invention.

Tin oxide sensors generally decrease in resistivity when theconcentration of the gas they are sensitive to increases. Exceptionallysensors of particular composition have been described which increase inresistivity as the concentration of a particular gas increases (see forexample references 22, 23 listed below).

Two of the inventors in the present application have also developed anddisclosed a linear-response hydrogen-selective tin oxide gas sensorincorporating bismuth (WO91/14939).

It has been proposed in U.S. Pat. No. 4,542,640 (Clifford) to provide agas detection system incorporating several separate semiconductor gassensors having different characteristics and processor means to producean indication of gas concentration.

In a first aspect of the invention the inventors have found that bypre-calcining the tin oxide in such sensors they can control thebehaviour of the sensor such that a sensor is obtained having widelydifferent characteristics at different operating temperatures. Such asensor can be used at several different temperatures and the resultscompared to obtain more accurate measurement of gas concentrations andcomposition than hitherto.

Alternatively several such sensors can be maintained at differenttemperatures so that their resistances can be measured simultaneously ina like manner to the U.S. Patent mentioned above.

Accordingly in a first aspect the present invention provides a tin oxidegas sensor having a resistivity that at a measuring temperatureincreases with concentration of at least one gas to be measured, thesensor being formed by calcining the tin oxide in air at a temperaturein excess of 1400° C., or having a state of physical aggregationconsistent with being formed in such manner.

The inventors believe that calcining at such temperatures alters thenature of the non-stoichiometry of the tin oxide, or possibly acts byother means affecting the state of physical aggregation of the tin oxidesuch as affecting the surface of the tin oxide particles. The inventorsbelieve that such alteration in the nature of the non-stoichiometry orstate of physical aggregation of the tin oxide may be creatable by othermeans such as firing in atmospheres with differing oxygen contents fromair or by the use of reducing agents such as tin.

The invention further provides a method for making the resistivity of atin oxide gas sensor at a measuring temperature increase with increasein the concentration of at least one gas to be measured by inducing inthe tin oxide non-stoichiometry or a state of physical aggregationconsistent with being formed by calcining the tin oxide in air at atemperature in excess of 1400° C.

Advantageously the calcining temperature is approximately 1500° C.

More advantageously, at a second measuring temperature the resistivityof the sensor to said one gas decreases with increasing gasconcentration.

Yet more advantageously the resistivity of the sensor is dependent onthe concentration of several gases, the dependence at differingmeasuring temperatures being such that by measuring the resistivity ofthe sensor at several different measuring temperatures the compositionof a gas to which the sensor is exposed may be calculated.

In a further aspect of the invention the inventors have found that thesusceptibility of tin oxide sensors to oxygen cross-sensitivity isdepressed if certain dopants are used to lower the initial baseresistance of the sensors.

The present applicants have found that if Sb(III) is included as adopant the sensitivity to oxygen concentration decreases dramatically.When such a sensor incorporating, say, 2% Sb₂ O₃, is exposed toconditions of reduced oxygen partial pressure (P_(O2) in the range 10⁻⁴-1 atm.) the sensitivity to these gases merely increases by up to afactor of times two and usually much less. The inventors initiallysurmised that the antimony worked by reducing base resistance of thesensor, however comparative tests with other potential dopants.indicates that this is not the only mechanism operating.

Accordingly in a further aspect the present invention provides a methodfor the production of tin oxide sensors comprising the incorporation ofantimony in the tin oxide in an amount sufficient to render thesensitivity of the sensor to one or more of the gases H₂, CO, or CH₄,relatively independent of the concentration of oxygen in the rangeP_(O2) 10⁻⁴ -1 atm.

Advantageously the concentration of antimony expressed as antimony oxidein the tin oxide is of the order of 2% w/w.

In yet a further aspect the invention provides a unitary sensorcomprising a plurality of tin oxide sensors on a common substrate atleast one of the sensors being an antimony doped sensor as describedabove and at least one other of the sensors being a high temperaturecalcined or like sensor as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the invention are exemplified by the followingdescription and claims, with reference to the drawings in which FIGS.1-9 refer to the investigation of non-stoichometry consistent with hightemperature calcining and FIGS. 10-13 refer to investigation ofsuppression of oxygen cross-sensitivity:

FIG. 1: Variation of sensor resistance as a function of SnO₂ sinteringtemperature at (a) T_(o) =400° C. and (b) T_(o) =280° C.

FIG. 2: Arrhenius-type plots of lnR versus T⁻¹ for a series of sensorsfabricated from SnO₂ samples preheated at the following temperatures:(i) 800° C., (ii) 1125° C., (iii) 250° C., (iv) 1440° C., (v) 1500° C.and (vi) 1580° C.

FIG. 3: Logarithmic plots of resistance versus reducing gasconcentration at a working temperature of 400° C., for a series ofsensors sintered at temperatures of (i) 800° C., (ii) 1000° C., (iii)1125° C., (iv) 1250° C., (v) 1375° C., (vi) 1440° C., (vii) 1500° C. and(viii) 1580° C. The contaminant gases used in each case are (a) CO, (b)CH₄ and (c) H₂.

FIG. 4: A plot of R_(o) /R (where R_(o) =sensor resistance in air and R=resistance in a 1% v/v contaminant gas/air mixture) versus sinteringtemperature (T_(S)) for a series of sensors maintained at T_(o) =400° C.

FIG. 5: As FIG. 3 except that a working temperature of 280° C. wasemployed. The reducing gases tested were (a) CO, (b) CH₄ and (c) H₂.

FIG. 6: Logarithmic plots of resistance response to reducing gasinclusions for a sensor prepared from SnO₂ sintered at 1500° C., andmaintained at working temperatures of (i) 450° C., (ii) 400° C., (iii)360° C., (iv) 320° C., (v) 280° C., (vi) 230° C. and (vii) 175° C. Thecontaminant gases used in each case are (a) CO, (b) CH₄ and (c) H₂.

FIG. 7: Dynamic response of a sensor fabricated from SnO₂ preheated at1500° C. to different CO concentrations in air. The operatingtemperature used is 280° C.

FIG. 8a: A plot of sensor response (represented by the ratio ofresistance in supporting gas to the resistance exhibited in the presenceof a 1% v/v reducing gas inclusion) versus oxygen partial pressure foran undoped SnO₂ sample sintered at 1500° C. upon exposure to (i) CO at280° C. and (ii) H₂ at 175° C.

FIG. 8(b): Sensor resistance response to reducing gas inclusions whenthe oxygen partial pressure in the base gas is fixed at 10⁻⁴ atm. Anoperating temperature of 280° C. was employed.

FIG. 9: Resistance versus contaminant gas concentration plots for asensor fabricated from SnO₂ pre-sintered at 1500° C. in air andmaintained at three different operating temperatures; (a) 400° C., (b)280° C. and (c) 175° C.

FIG. 10 is a graph indicating the response of a series of Sb₂ O₃ dopedtin oxide sensors (measured as G/G_(o), where G=conductance in acontaminant--O₂ --N₂ mixture and G₂ =conductance in supporting gas only)to a 1% v/v carbon monoxide inclusion plotted as a function of oxygenpartial pressure.

FIG. 11 as FIG. 10 except that a 1% v/v methane inclusion was used.

FIG. 12 as FIG. 10 except that a 1% v/v hydrogen inclusion was used.

FIG. 13 Response (G/G₀) of a CO selective sensor doped with 2% w/w Sb₂O₃ plotted as a function of oxygen partial pressure. A contaminant gasconcentration of 1% v/v was used in each case while the sensor wasmaintained at an operating temperature of 280° C.

INVESTIGATION OF EFFECT OF SINTERING TEMPERATURE

Stannic oxide was prepared via the controlled hydrolysis of an aqueousSnCl₄ solution by urea at 90° C. The gelatinous precipitate obtained waswashed thoroughly with distilled water until the chloride concentrationin the filtrate became negligible. After drying, heat treatment of theα-stannic acid in air at 800° C. for 2 hours ensured complete conversionto tin (IV) oxide. A fine homogeneous powder was obtained by grindingthe oxide in a ball mill for 30 minutes.

Sensors were prepared by applying an aqueous paste of the SnO₂ acrossthe contact pads of an alumina substrate (supplied by RosemountEngineering) as described in previous publications [13, 14]. Forsintering temperatures of 800° C. to 1000° C., the whole tindioxide/substrate assembly was placed directly in the furnace in air.However, due to the inability of the substrate to withstand temperaturesexceeding 1000° C., heat treatment (calcining) of the SnO₂ between 1100°C. and 1600° C. was performed on the free powder in air. The precalcinedoxide was then applied to the substrate and fired at 1000° C. in theusual manner.

Full details of the procedure adopted for determining sensor resistanceand blending mixtures of CO, CH₄ or H₂ in an oxygen-nitrogen supportinggas are given elsewhere [13, 14, 19].

A study of the sensor resistance versus working temperature (T_(o))relationship in clean dry air shows that increasing the SnO₂ sinteringtemperature leads to a substantial rise in resistance. FIG. 1illustrates the change in sensor resistance observed at two differentvalues of T_(o) for SnO₂ samples sintered between 800° C. and 1600° C.The greatest increases in resistance are exhibited upon sintering thematerial at temperatures in excess of 1400° C. This result appears toconflict somewhat with the findings of Sasaki et al [12] who observed afall in sensor resistance at sintering temperatures in excess of 1300°C. which they ascribe to the formation of `necks` between separate oxideparticles.

Arrhenius type treatment of the data obtained for several sensorsemploying a range of sintering temperatures are shown in FIG. 2[Sintering temperatures are:--(i) 800° C., (ii) 1125° C., (iii) 1250°C., (iv) 1440° C., (v) 1500° C. and (vi) 1580 ° C]. A curiouscharacteristic of these plots is the gradual appearance of an inflectionin the resistance-temperature curve between 230° C. and 350° C. as T_(S)is increased. Such behaviour has been observed in previous studies onthe electrical conductance of pressed porous pellets of SnO₂ sintered at1000° C. [17] and attributed to a change in the absorbed oxygen speciespresent on the sensor surface. However, the inflection manifests itselfat a temperature which is significantly higher than the well establishedvalue of 160° C. determined for the O₂ ⁻ →20⁻ transformation [19].

The slopes of the lnR versus T⁻¹ plots for working temperature exceeding350° C. vary only marginally with sintering temperature (T_(S)) as shownin Table 1.

                  TABLE 1                                                         ______________________________________                                        Effect of sintering temperature on the slopes of Arrhenius                    plots obtained in the 320° C. to 500° C. region for             undoped                                                                       SnO.sub.2 sensors.                                                            Sintering Temperature/°C.                                                                Arrhenius slope/eV                                          ______________________________________                                         800              0.95                                                        1000              0.99                                                        1125              0.90                                                        1250              0.90                                                        1375              0.85                                                        1440              0.74                                                        1500              0.91                                                        1580              0.95                                                        ______________________________________                                    

The high temperature activation energy determined for a SnO₂ sensorsintered at 1000° C. corresponds well with the findings of Moseley et al[17]. From their results these authors deduce that a surface stateassociated with adsorbed oxygen is located at 1.1 eV below theconduction band,.

Many authors have observed that the SnO₂ grain size increases withincreasing values of T_(S) and that a rise in sensor resistance alsoensues [8, 12, 20]. Sasaki and his co-workers [12] have assigned thisincrease in resistance to the elimination of shallow donor levels as thesintering temperature is increased. However, it may also be possiblethan an enlargement in the crystallite size leads to a decrease in thenumber of intergrain boundaries thus restricting the flow of carriersthrough the sintered mass of material.

FIG. 3(a)-(c) illustrates the variation of sensor resistance at ≈400° C.as a function of contaminant gas concentration for a series of SnO₂samples sintered in the range 800° C.<T_(S) >1600° C. [Sinteringtemperatures are:--(i) 800° C., (ii) 1000° C., (iii) 1125° C., (iv)1250° C., (v) 1375° C., (vi) 1440° C., (vii) 1500° C. and (viii) 1580°C.]. The main characteristics of sensor response to each reducing gastested will be discussed in turn.

(a) CO response

Sensors sintered at 800° C. or 1000° C. appear to possess little or noresponse to carbon monoxide at operating temperatures equal to orexceeding 400° C. However, increasing T_(S) beyond 1100° C. appears toconfer a degree of CO sensitivity which reaches a maximum at circa 1250°C. A sensor fabricated from a SnO₂ sample pretreated at this temperatureexhibits a 66% resistance drop upon exposure to a 1% v/v CO/air mixture.Further increasing T_(S) diminishes the CO signal to a certain extent,yet the response even at the highest sintering temperature employedremains significant.

(b) CH₄ response

The use of the lowest sintering temperatures yield sensors possessinghigh sensitivity to methane. An increase in T_(S) leads to a gradualdecline in the CH₄ signal, with the consequence that the SnO₂ samplesintered at the highest temperature generates the least sensitiveelement.

(c) H₂ response

The magnitude of resistance changes observed upon exposure to hydrogeninclusion for sensors employing T_(S) ≦51000° C. appear relativelyinsubstantial compared to the response exhibited in the presence of anequally concentration of methane. However, increasing the temperature ofsintering initiates a substantial hydrogen sensitivity which reaches amaximum at around 1400° C. The use of higher values of T_(S) leads tothe desensitisation of the H₂ signal.

The trends described above are adequately represented by FIG. 4 wherethe ratio R_(o) /R (where R_(o) =sensor resistance in clean air andR=sensor resistance in a 1% v/v contaminant/air mixture) is plotted as afunction of sintering temperature. Therefore, if an undoped tin oxidesensor of this type is to be operated at a relatively high temperature(T_(o) =400° C.) then the greatest degree of selectivity to a certaingas, namely methane, is achieved by employing a sintering temperature inthe range 800° C.-1000° C. However, should sensitivity to a range ofreducing gases be the main requirement, the use of T_(S) in the range1250° C.-1400° C. would be most advantageous.

The logarithmic plots of sensor resistance versus contaminant gasconcentration illustrated in FIG. 3(a)-(c) indicate that a power lawrelationship is obeyed by the majority of sensors. However, sensorresponse, especially to carbon monoxide, appears in several cases merelyto asymptotically approach the power law relationship at highcontaminant levels. Similar characteristics have been reported by others[21] during studies of the stead state gas response of TGS semiconductorgas sensors. The power law coefficient (β) varies considerably withsintering temperature as can be ascertained from the data presented inTable 2(a). Again the general trends in the magnitude of β mirror thoseobserved for the variation of CO, CH₄ and H₂ sensitivity with T_(S) asdiscussed above.

                  TABLE 2                                                         ______________________________________                                        Variation of β, the power law slope as a function of                     sintering temperature for undoped SnO.sub.2 sensors maintained                at two different working temperatures upon exposure to                        inclusion of CO, CH.sub.4 or H.sub.2 in air.                                  ______________________________________                                        (a)T.sub.o = 400° C.                                                   Sintering Temperature/°C.                                                               β(CO)                                                                              βCH.sub.4)                                                                       βH.sub.2)                             ______________________________________                                         800             0         0.49    0.30                                       1000             0         0.54    0.35                                       1125             0.10      0.37    0.45                                       1250             0.32      0.47    0.54                                       1375             0.25      0.34    0.54                                       1440             0.36      0.36    0.45                                       1500             0.22      0.33    0.39                                       1580             0.18      0.36    0.44                                       ______________________________________                                        (b)T.sub.o = 280° C.                                                   Sintering Temperature/°C.                                                               β(CO)                                                                              βCH.sub.4)                                                                       βH.sub.2)                             ______________________________________                                         800             0.38      0.46    0.95                                       1000             0.40      0.35    0.86                                       1125             0.25      0.39    0.68                                       1250             0.19      0.43    0.47                                       1375             0.15      0.22    0.53                                       1440             -0.10*    0.12    0.34                                       1500             -0.72*    0.06     0.51*                                     1580             -0.14*    0.04    0.22                                       ______________________________________                                         *In cases where two types of behaviour are exhibited by the resistance        versus gas concentration plots, the slope obtained in the 10.sup.2            -10.sup. 3 ppm region is displayed.                                      

Identical experiments to those described above were performed at a lowerworking temperature of 280° C. This temperature was establishedpreviously [18] as the optimum required for maximum sensitivity tocarbon monoxide and hydrogen. The plots of sensor resistance versuscontaminant gas concentration obtained for the series of sensorsfabricated from SnO₂ samples pre-heated over a range of temperatures areshown in FIG. 5(a)-(c). Sensor properties at T_(o) =280° C. differ inseveral ways to those exhibited at a high working temperature.

(a) CO response

A significant decline in carbon monoxide sensitivity is observed uponincreasing the temperature of sintering from 800° C. to 1375° C. Sensorsfabricated from SnO₂ heat treated at temperatures equal to or exceeding1440° C. display p-type behaviour upon exposure to CO inclusion of lessthan 10³ ppm. The most magnified resistance rises are observed when asintering temperature of 1500° C. is employed. Increasing the COconcentration above 10³ ppm in this case causes diminution and eventualcessation of the resistance increments observed.

(b) CH₄ response

A decline in methane sensitivity with increasing temperature isdisplayed at the lower operating temperature, in accordance with theresults attained at T_(o) ≈400 C. However, at pretreatment temperaturesof 1500° C. or above, CH₄ response becomes negligible.

(c) H₂ response

The use of the lowest sintering temperatures confers a substantialhydrogen sensitivity, which decreases as T_(S) is raised. Curiously, asensor fabricated from an SnO₂ sample sintered at 1500° C. exhibitsconventional n-type behaviour upon exposure to hydrogen concentrationsof less than 10³ ppm, yet as H₂ levels are increased further, sensorresistance rises significantly. This anomaly is not displayed by thesensor sintered at 1580° C.

Table 2(b) shows the effect of sintering temperature on B, the power lawcoefficient calculated from the plots illustrated in FIG. 5(a)-(c), at asensor working temperature of 280° C. Again the variation of β withsintering temperature is very substantial especially in the case of COwhere a switch from a positive to a negative power law coefficient isobserved at T_(S) ≈1440° C. It can be seen that the magnitude of thepower law slope for H₂ gas is approximately a factor of 2 greater thanfor CO or CH₄ at sintering temperatures of less than 1125° C. Thisfinding concurs with previously published results [21].

The gas sensing properties of polycrystalline SnO₂ sintered at 1500° C.were studied in view of its switch from n-type to p-type behaviourdepending upon the conditions employed. The vast discrepancies in sensorresponse to reducing gases at the two operating temperatures utilisedabove (≈400° C. and ≈280° C.) indicate that temperature is crucial indetermining the behaviour of the sensor. The results of an exhaustivestudy of the effect of the variation of T_(o) upon sensor response isrepresented in FIG. 6(a)-(c) where sensor resistance is plotted as afunction of contaminant gas concentration using logarithmic axes[Working temperatures of (i) 450° C., (ii) 400° C., (iii) 360° C., (iv)320° C., (v) 280° C., (vi) 230° C. and (vii) 175° C.].

If maintained at temperatures of 360° C. or above, the sensorexperiences a drop in resistance upon exposure to CO, CH₄ or H₂inclusions in air, as would be expected for this type of device.Decreasing T_(o) below 360° C. produces several effects. Firstly, theresistance changes observed upon exposure to methane become negligibleat working temperatures of 280° C. or less. However, secondly and morestriking is the change in the mechanism of detection of CO exhibited bythe sensor, where substantial increases in resistance are observed inthe presence of CO/air mixtures. The magnitude of the p-type responsereaches a maximum at T_(o) ≈230° C., where a 16 fold rise in sensorresistance is experienced in a 1% v/v CO atmosphere. Lowering theworking temperature further leads to a severe reduction in the size ofthe p-type effect. A similar `reverse sensitivity` is also exhibitedupon exposure to hydrogen-containing environments at workingtemperatures of 280° C. or less. In this case the most enhanced increasein resistance is observed at the lowest value of T_(o) used.

The dynamic response of the sensor maintained at T_(o) ≦280° C. appearssubstantially slower than the n-type signal observed at high operatingtemperatures. In the latter instance, the final resistance reading isreached 30 seconds or less after exposure to the reducing gas. However,at T_(o) ≈280° C. the resistance versus time profiles plotted in FIG. 7shows that the final signal is only attained circa 5 minutes afterintroduction of the CO/air mixture. Recovery of the original resistancein air appears even slower, taking up to 30 minutes when the sensor haspreviously been dosed with carbon monoxide concentrations of 0.5% v/v orgreater.

The reproducibility of the phenomenon for several sensors prepared fromdifferent batches of SnO₂ sintered at 1500° C. was investigated. Theseexperiments revealed that this p-type behaviour was displayed in allcases, but the magnitude of the observed resistance increases for afixed CO or H₂ concentration varied significantly between tin oxidebatches.

In a separate investigation, the effect of different oxygen partialpressures on sensor response at operating temperatures of 280° C. orless were studied. FIG. 8(a) illustrates the change in the ratio R/R_(o)(defined previously) for a 1% v/v CO inclusion as the oxygen partialpressure (P_(o2)) is decreased from 1 to 10⁻⁴ atm. A steady decline inthe p-type response is observed as P_(o2) is lowered in supporting gasonly i.e. a conventional n-type detection mechanism. The results of asimilar study of the oxygen dependence of H₂ response at T_(o) =175° C.are also included in FIG. 8(a). Here the resistance increment observedupon dosing with 1% v/v H₂ inclusions remains constant at P_(o2) isdecreased from 1 to 10⁻² atm. However, further reduction of P_(o2)results in the curtailment of the signal until changes becomeinsignificant when the oxygen partial pressure is 10⁻⁴ atm.

FIG. 8(b) shows the sensor response characteristics at T_(o) =280° C.when P_(o2) in the base gas is fixed at 10⁻⁴ atm. and the reducing gasconcentration is varied. As can be seen from the resistance versus [CO]or [H₂ ] plots obtained, sensor behaviour has reverted to conventionaln-type, while sensitivity to CH₄ remains virtually negligible. Theresistance response of the sensor to oxygen in the absence of reducinggas revealed that a power law relation exists. A value of 0.29 wasdetermined for B, the power law slope from a logarithmic plot of sensorresistance versus P_(o2).

How exactly the observed change in the SnO₂ gas detection mechanismarises is unclear. Other researchers [22, 23] have observed similarphenomena during their investigation of ThO₂ --or ZrO₂ - doped tin oxidesensors sintered at 600° C. or 800° C. respectively. An increase insensor resistance occurs as the ThO₂ --added SnO₂ sample is exposed toappropriate concentrations of hydrogen at temperatures of 220° C. orlower [22], while the ZrO₂ --doped material acts similarly in thepresence of ammonia [23]. Kanefusa et al [22] tentatively suggests thatthis negative sensitivity is caused by a change in adsorption rates orphysical nature of the adsorbates on the sensor surface under theseconditions. It may be possible that the change in response is due to ahigh temperature calcining induced change in the nature of thenon-stoichiometry of the tin oxide resulting in differing chargecarriers being responsible for high and low temperature conductivity. Itmay be possible that preheating undoped SnO₂ at a temperature of 1500°C. modifies the sensor surface to a considerable extent thus allowingsuch changes to occur.

Previously reported studies of the effects of thermal pretreatment onthe properties of SnO₂ [24, 25] have concentrated for the most part onthe catalytic activity of tin oxide annealed in the 200° C. to 800° C.region in mediating a range of oxidation reactions. However, a morefundamental investigation by Goodman and Gregg [8] shows thatsignificant charges occur to SnO₂ upon high temperature sintering. Theseauthors report that the specific surface area of stannic oxide ismarkedly reduced upon increasing the calcination temperature from 250°C. to 1400° C., but this is only accompanied by a minimal change rangein pore volume. However, at a temperature of circa 1550° C. the porevolume suddenly falls to zero, an occurrence that is liable to have acritical influence on the sensing properties of a device fabricated fromsuch a material.

A more detailed study of the properties of this sensor entailed thedetermination of response to other reducing gases such as methane orhydrogen over a range of working temperatures. The results obtained aresummarised in FIG. 9 where the variation of sensor resistance as afunction of CO, CH₄ or H₂ concentration is monitored at operatingtemperatures of 400° C., 290° C. and 175° C. respectively. As would beexpected for this type of n-type semiconducting material, at workingtemperatures of 360° C. or above the sensor experiences a drop inresistance upon exposure to each contaminant gas tested. However, shouldthe device be operated at temperatures below 360° C. several changes insensor behaviour are manifested. Firstly, the resistance changesobserved in the presence of methane inclusions become negligible atworking temperatures of 280° C. or less. However, secondly and morestriking is the change in the mechanism of CO exhibited by the sensorwhere substantial increases in resistance are observed in the presenceof carbon monoxide/air mixture.

The magnitude of the p-type response reaches a maximum at 230° C. wherea 16 fold rise in resistance is experienced in a 1% v/v CO atmosphere.Further reduction of the operating temperature leads to a similarreverse sensitivity to H₂ as shown in FIG. 2(c), but a severe decreasein the p-type effect observed for carbon monoxide.

The inventors' studies have revealed that sensor characteristics varyconsiderably with the SnO₂ pre-treatment temperature. The use of T_(S)=800° C. or 1000° C. confers greatest methane sensitivity at high sensorworking temperatures and maximum CO or H₂ response for T₂ =280° C. If asensor is to be maintained at circa 400° C. then the greatest hydrogenand carbon monoxide sensitivity is obtained by utilising sinteringtemperatures in the 1250° C.-1400° C. range.

A sensor prepared from a tin oxide sample fired at 1500° C. displayedsome remarkable properties. If this type of device is operated at 400°C. then the resistance of the sensor decreases in the presence of CO,CH₄ or H₂. However, the use of lower working temperatures leads tonegative sensitivity to CO and H₂, i.e. sensor resistance increasessignificantly upon exposure to the reducing gas. Such anomalousbehaviour may have important selectivity implications. A sensormaintained at 280° C. displays p-type response to CO, conventionaln-type detection of H₂ and negligible sensitivity to CH₄. Therefore, asingle sensor capable of discerning between different reducing gases ifoperated at several pre-set temperatures can be generated merely byutilising a high temperature sintering step prior to fabrication.

The p-type carbon monoxide or hydrogen response occurs despite apparentn-type semiconductivity revealed by resistance-temperature andresistance-oxygen partial pressure relationships. It is also known thatthe phenomenon is only exhibited in the presence of oxygen containingenvironments when P_(o) exceeds 10⁻³ atm.

INVESTIGATION OF EFFECT OF SENSOR BASE RESISTANCE & DOPANTS

Tin oxide sensors were made for experimental purposes from an aqueousslurry containing SnO₂, Sb₂ O₃ (1-3% w/w). No metal catalyst was addedor necessary. The SnO₂ was made by hydrolysis of SnCl₄ (BDH `Analar`grade) and ground with an appropriate amount of Sb₂ O₃ (Johnson MattheyChemicals `Specpure` grade) typically to <15 μm. A bead of this pastewas then applied across platinum contact pads deposited on an aluminasubstrate (Rosemount) left to dry in air and then sintered at 1000° C.for 2 hours (using a furnace heating rate of 400° C./hour).

Illustrated in FIG. 10 is the CO response of several Sb doped sensorsplotted as a function of oxygen partial pressure. The response in thiscase is represented by the ratio G/G_(o) where G_(o) =sensor conductancein the O₂ /N₂ base mixture and G=conductance observed in the presence ofa 1% v/v contaminant gas inclusion. The data presented show that even avery small quantity of added Sb₂ O₃ (0.2% w/w) reduces the observedresponse at the lowest oxygen concentration tested by over 2 orders ofmagnitude. Further increase of the sensor Sb₂ O₃ content leads to thecomplete elimination of any increase in CO response as P_(O2) isdecreased in the range 1-10⁻⁴ atm. This effect is also observed forother reducing gases such as CH₄ (FIG. 11) and hydrogen (FIG. 12). Adirect comparison of sensor response in two different oxygen containingenvironments is shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        A comparison of the response of a series of Sb.sub.2 O.sub.3 doped            tin dioxide sensors observed in air and under conditions                      of greatly reduced oxygen partial pressure.                                   ______________________________________                                        Environment 1:  Air (P.sub.O.sbsb.2 - 0.21 atm.)                              % Sb.sub.2 O.sub.3 (w/w)                                                                 *G/G.sub.O (CO)                                                                          G/G.sub.O (CH.sub.4)                                                                      G/G.sub.O (H.sub.2)                         ______________________________________                                        0          4.80       3.57        4.60                                        0.2        4.64       3.40        14.0                                        0.5        3.21       2.21        5.44                                        1          4.10       2.48        3.99                                        2          2.76       1.53        4.62                                        3          2.33       1.72        4.20                                        ______________________________________                                        Environment 2:  P.sub.O.sbsb.2 - 10.sup.-4 atm., balance N.sub.2              % Sb.sub.2 O.sub.3 (w/w)                                                                 *G/G.sub.O (CO)                                                                          G/G.sub.O (CH.sub.4)                                                                      G/G.sub.O (H.sub.2)                         ______________________________________                                        0          6400       230         6800                                        0.2        45         35          88                                          0.5        77         15          76                                          1          17         5.3         17                                          2          3.72       1.67        3.94                                        3          2.93       1.74        4.64                                        ______________________________________                                         *Response in this type of study is represented by the ratio of sensor         conductance in the presence of a 1% v/v contaminant gas inclusion to the      conductance in supporting gas only.                                      

The data listed for each reducing gas imply that a 2% w/w inclusion ofSb₂ O₃ is sufficient to suppress any enhanced sensitivity at P_(O2)=10⁻⁴ atm. while maintaining a reasonably substantial signal in air,e.g. a value of G/G_(O) =4 observed in the presence of a 1% v/v H₂inclusion corresponds to a 75% drop in resistance.

The reasons for the action of Sb₂ O₃ in suppressing oxygen dependence ofsensor response remain unclear at present. It may be that the largenumber of electrons promoted from shallow Sb (III) donor levelseventually saturate the SnO₂ conduction band and conceal to a largeextent any substantial increases in conductivity.

Some support for this suggestion can be obtained by comparing finalresistance values for SnO₂ and Sb-doped sensors upon exposure to 1% v/vreducing gas at P_(O2) =10⁻⁴ atm. (Table 5).

                  TABLE 5                                                         ______________________________________                                        [Sb.sub.2 O.sub.3 ] w/w                                                                  R.sub.CO /kΩ                                                                        R.sub.CH.sbsb.4 /kΩ                                                              R.sub.H.sbsb.2 /kΩ                      ______________________________________                                        0          1.5         2.4      1.0                                           0.2        0.4         1.2      0.5                                           0.5        0.6         0.6      0.2                                           1          0.9         3.0      1.0                                           3          0.6         1.6      0.4                                           ______________________________________                                    

The variation between the resistance values listed above for each sensorappear small compared with the differences in original base resistance.It seems therefore that a point is reached beyond which sensorresistance cannot decrease any further, i.e. a minimum where theconduction band of the semiconductor is saturated.

The inventors attempted to test whether the low base resistance of theantimony doped sensors was the cause of their low oxygencross-sensitivity. A series of additives were used on test sensorshaving the same geometry and the effect of these additives aresummarised in Table 6 and 7 below.

Two different methods and media were used for applying the SnO₂ to formsensors. In the first method a tin oxide sample (prepared previously viathe hydrolysis of SnCl₄ [BDH `Analar` grade]) was ground in an aqueouspaste along with the appropriate quantity of Sb₂ O₃ typically to <15 μm.The Sb₂ O₃ used for these experiments was obtained from Johnson MattheyChemicals `Specpure` grade. The paste was then applied across theelectrode array of a gas sensor substrate, allowed to dry and thensintered in air at 1000° C. for 2 hours with additional two hour heatingand cooling ramps.

When a tin oxide from Keeling & Walker (`Superlite` grade) was used inthe above method it was found that cracking of the tin oxide resulted ina deficient sensor being formed. (The Keeling & Walker tin oxidecontains significant amounts of antimony oxide and other tracematerials). This defect could be avoided by forming the sensor in anon-aqueous medium. In this case the SnO₂ --Sb₂ O₃ mixture was ground inan inorganic media consisting of alpha-terpineol containing an ethylcellulose (1% w/w) stabiliser. The mixture was ground to typically <15μm. The paste was then applied to the substrate surface and dried for 1hour at a temperature of 80° C. The sensor was then sintered using theconditions described above at 1000° C.

An advantage of forming the sensor from a slurry in the terpineol baseis that by grinding to <15 μm a paste results which can be screenprinted and this is useful in preparation of large numbers of sensors.

                  TABLE 6                                                         ______________________________________                                               Medium of         Base      Resistance                                        application       resistance                                                                              when                                       Additive                                                                             to substrate                                                                            Power   (Air)     P.sub.o2 = 10.sup.-4                       ______________________________________                                                                           atm                                        (1)SnO.sub.2                                                                         Alpha-    3.27 W  0.44  kohm  0.23  kohm                               (none) terpineol                                                                     base                                                                   (2)SnO.sub.2                                                                         Aqueous   3.04 W  0.34  kohm  0.12  kohm                               (none) paste                                                                  (3)MoO.sub.3                                                                         Aqueous   3.29 W  2.90  kohm  1.75  kohm                                      paste                                                                  (4)WO.sub.3                                                                          Aqueous   3.24 W  4.40  kohm  1.85  kohm                                      paste                                                                  (5)Nb.sub.2 O.sub.5                                                                  Alpha-    3.23 W  78    kohm  31    kohm                                      terpineol                                                                     paste                                                                  (6)Sb.sub.2 O.sub.3                                                                  Aqueous   3.47 W  9.1   ohm   8.0   ohm                                       paste                                                                  (7)P.sub.2 O.sub.5                                                                   Aqueous   3.44 W  0.74  kohm  0.28  kohm                                      paste                                                                  (8)In.sub.2 O.sub.3                                                                  Aqueous   3.12 W  2200  kohm  510   kohm                                      paste                                                                  (9)Ta.sub.2 O.sub.5                                                                  Alpha-    3.23 W  83    ohm   40    ohm                                       terpineol                                                                     paste                                                                  (10)B.sub.2 O.sub.3                                                                  Aqueous   3.32 W  1.0   kohm  0.44  kohm                                      paste                                                                  (11)RuO.sub.2                                                                        Alpha-    3.27 W  1.45  kohm  0.68  kohm                                      terpineol                                                                     paste                                                                  ______________________________________                                    

                  TABLE 7                                                         ______________________________________                                                           Sensitivity                                                                             Sensitivity                                                Test     (R.sub.o /R.sub.gas)                                                                    (Ro/R.sub.gas) when                              Additive  gas      in Air    P.sub.o2 = 10.sup.-4 atm                         ______________________________________                                        (1)SnO.sub.2                                                                            CO       4.22      8.13                                             (none)    CH.sub.4 2.75      7.72                                                       H.sub.2  8.52      28.0                                             (2)SnO.sub.2                                                                            CO       4.73      9.58                                                       CH.sub.4 3.33      5.30                                                       H.sub.2  7.81      14.1                                             (3)MoO.sub.3                                                                            CO       2.07      27.1                                                       CH.sub.4 2.20      16.7                                                       H.sub.2  4.25      174                                              (4)WO.sub.3                                                                             CO       2.21      50                                                         CH.sub.4 1.69      12.5                                                       H.sub.2  4.19      108                                              (5)Nb.sub.2 O.sub.5                                                                     CO       10.6      196                                                        CH.sub.4 4.16      41                                                         H.sub.2  17.4      375                                              (6)Sb.sub.2 O.sub.3                                                                     CO       1.46      1.41                                                       CH.sub.4 1.22      1.15                                                       H.sub.2  1.39      1.38                                             (7)P.sub.2 O.sub.5                                                                      CO       2.45      14.0                                                       CH.sub.4 2.39      7.58                                                       H.sub.2  13.5      15.7                                             (8)In.sub.2 O.sub.3                                                                     CO       8.15      1800                                                       CH.sub.4 3.11      13.9                                                       H.sub.2  68.6      12500                                            (9)Ta.sub.2 O.sub.5                                                                     CO       2.14      4.15                                                       CH.sub.4 1.69      2.88                                                       H.sub.2  3.79      5.67                                             (10)B.sub.2 O.sub.3                                                                     CO       6.47      33                                                         CH.sub.4 5.20      18.8                                                       H.sub. 2 20.5      46.3                                             (11)RuO.sub.2                                                                           CO       4.05      12.9                                                       CH.sub.4 2.31      5.0                                                        H.sub.2  6.59      23.2                                             ______________________________________                                          to a large decrease in base resistance, comparable to that induced by     antimony. However it is clear from Table 7 that despite this the ratio of     sensitivity in air to sensitivity at P.sub.O2 =10.sup.-4 remains close to     that for undoped tin oxide.

In a separate experiment, to ascertain whether a CO sensor could be madehaving reduced oxygen cross-sensitivity, the behaviour of a sensorcomposed of SnO₂ (83%), Bi₂ O₃ (15%) and Sb₂ O₃ (2% w/w) in differentoxygen containing environments was studied. Thins sensor was found to beCO and H₂ selective having a negligible response to CH₄. FIG. 13illustrates the variation of sensor response to 1% v/v CO, CH₄ and H₂inclusions as a function of oxygen partial pressure. In contrast to theSnO₂ --Sb₂ O₃ (2% w/w) system, this sensor exhibits significantincreases in CO and H₂ response when PO2 is reduced below 10⁻³ atm.However, one advantage is that selectivity to these gases isretained[over a wide oxygen concentration range and CH₄ response remainsnegligible. The different behaviour in the presence or absence of Bi₂ O₃may perhaps be associated with the dissimilarity in the resistivities ofboth systems. SnO₂ reacts with Bi₂ O₃ at temperatures in excess of 600°C. to form Bi₂ Sn₂ O₇, and this is accompanied by the consumption of alarge number of conduction band electrons with the consequence that alarge increase in resistance is observed. Though relatively highconductivity is restored upon the addition of Sb₂ O₃ by promotion ofelectrons from Sb (III) donor levels, the occupancy of the conductionband is not sufficiently great to enable concealment of any largechanges in sensor conductance.

This aspect of the invention does not include systems in which Bi₂ O₃ or,other materials are present which react with antimony to prevent itseffect of suppressing oxygen cross-sensitivity.

Combined Unitray Sensor

From the above it will appear clear that the high temperaturepre-calcined sensor exhibits a useful variance in behaviour withtemperature such that it can be used at varying temperatures (either byvarying the temperature of one sensor or using several sensors held atdifferent temperatures) so as to enable determination of several gases.Indeed if several sensors are used they may advantageously be formed ona single substrate having separate heaters for each sensor.

The high temperature pre-calcined sensor shows oxygen cross-sensitivityat low oxygen concentrations (<10⁻³ atm.) and so if an antimony dopedsensor is also applied to the same substrate a comparison between thesignal for a given as from the pre-calcined sensors and the antimonydoped sensor will enable one to deduce the oxygen partial pressure.

Indeed it would also be possible to apply a linear-response hydrogenelectrode to the same substrate.

Conventional circuit design principles will apply to determining themost appropriate lay-out of the sensors on the substrate but care mustbe taken to ensure that at the sintering stage elements such as bismuthand antimony do not migrate.

REFERENCES

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We claim: PG,31
 1. A method for the production of tin oxide sensorscontaining Sb (III) as a dopant in oxide form, the method comprisingmixing antimony bearing material with tin oxide powder and forming thesensor by depositing a slurry of the mixture onto a substrate and dryingand sintering the slurry, the antimony bearing material being present inan amount sufficient to render the sensitivity of the sensor to one ormore of the gases H₂, CO, or CH₄, substantially independent of theconcentration of oxygen in the range P₀₂ 10⁻⁴ -1 atm.
 2. A method asclaimed in claim 1 in which the antimony is in the form of antimonyoxide powder.
 3. A method as claimed in claim 2 in which theconcentration of antimony is of the order of 2% w/w expressed asantimony oxide in the tin oxide.
 4. A method of making a tin oxide gassensor having a resistivity that at a measuring temperature increaseswith concentration of at least one gas to be measured, comprising thestep of calcining the tin oxide in air at a temperature in excess of1400° C., or otherwise heat treating the tin oxide so that it has astate of physical aggregation consistent with being formed by calciningin air at a temperature in excess of 1400° C.
 5. A method as claimed inclaim 4 in which the calcining temperature is approximately 1500° C. 6.A tin oxide gas sensor having a resistivity that at a measuringtemperature increases with concentration of at least one gas to bemeasured, the sensor being made by the method of claim
 4. 7. A tin oxidegas sensor comprising an inhomogeneous mixture of tin oxide and antimonyin the form of Sb (III) as a dopant in oxide form.
 8. A tin oxide gassensor as claimed in claim 4 in which the concentration of antimony isof the order of 2% w/w expressed as antimony oxide in the tin oxide. 9.A tin oxide gas sensor as claimed in claim 8 in which at a secondmeasuring temperature the resistivity of the sensor to said one gasdecreases with increasing gas: concentration.
 10. A tin oxide gas sensoras claimed in claim 6 in which the resistivity of the sensor isdependent on the concentration of several gases, the dependence atdiffering measuring temperatures being such that by measuring theresistivity of the sensor at several different measuring temperaturesthe composition of a gas to which the sensor is exposed may becalculated.
 11. An array of tin oxide gas sensors mounted on a singlesubstrate comprising a plurality of sensors as claimed in claim 6 withheater means to maintain the sensors at differing temperatures.
 12. Anarray of sensors as claimed in claim 11 comprising additionally a sensorcomprising an inhomogeneous mixture of tin oxide and antimony bearingmaterial.
 13. An array of sensors as claimed in claim 11 comprisingadditionally a linear-response hydrogen-selective tin oxide gas sensorincorporating bismuth.